U.S. patent application number 10/384978 was filed with the patent office on 2004-08-19 for optical systems including add drop devices and methods.
This patent application is currently assigned to Corvis Corporation. Invention is credited to Stephens, Thomas D..
Application Number | 20040161237 10/384978 |
Document ID | / |
Family ID | 26797825 |
Filed Date | 2004-08-19 |
United States Patent
Application |
20040161237 |
Kind Code |
A1 |
Stephens, Thomas D. |
August 19, 2004 |
Optical systems including add drop devices and methods
Abstract
Optical systems of the present invention include an add/drop
device and/or cross-connect device, which is reconfigurable to add
and drop signal wavelengths between one or more transmission paths.
The add/drop device includes a first selective element configurable
to pass a first group of signal wavelengths including at least a
first signal wavelength from an input path to a first add/drop path
and pass continuing signal wavelengths differing from the first
group of signal wavelength to a second add/drop path. A second
selective element is provided that is configurable to pass a second
group of signal wavelengths including at least a second signal
wavelength from said first add/drop path and continuing signal
wavelengths differing from the second group of wavelengths from
said second add/drop path to an output path. The second selective
element is further configured to pass the second group of signal
wavelengths from said second add/drop path and wavelengths
differing from the second wavelength from said first add/drop path
to a drop path. The add/drop device may also include an add path to
introduce add signal wavelengths directly to the output path or via
at least one of the first and second selective elements.
Inventors: |
Stephens, Thomas D.;
(Columbia, MD) |
Correspondence
Address: |
CORVIS CORPORATION
INTELLECTUAL PROPERTY DEPARTMENT
7015 ALBERT EINSTEIN DRIVE
COLUMBIA
MD
210469400
|
Assignee: |
Corvis Corporation
ATTN: Intellectual Property Department 7015 Albert Einstein
Drive, P.O. Box 9400
|
Family ID: |
26797825 |
Appl. No.: |
10/384978 |
Filed: |
March 10, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10384978 |
Mar 10, 2003 |
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09398541 |
Sep 17, 1999 |
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6538783 |
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60101031 |
Sep 18, 1998 |
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Current U.S.
Class: |
398/82 |
Current CPC
Class: |
H04J 14/0204 20130101;
H04J 14/0205 20130101; H04J 14/0206 20130101; H04J 14/021 20130101;
H04J 14/0213 20130101 |
Class at
Publication: |
398/082 |
International
Class: |
H04J 014/02 |
Claims
What is claimed is:
1. An optical system comprising: at least a first optical
transmitter configured to transmit information via optical signals
including at least one signal wavelength; an add/drop device
configured to receive the optical signal from said first optical
transmitter via an input path, said add/drop device including a
first selective element configurable to pass a first signal
wavelength from said input path to a first add/drop path and pass
continuing signal wavelengths differing from the first signal
wavelength to a second add/drop path, a second selective element
configurable to pass a second signal wavelength from said first
add/drop path and continuing signal wavelengths differing from the
second signal wavelength from said second add/drop path to an
output path and pass the second signal wavelength from said second
add/drop path and signal wavelengths differing from the second
signal wavelength from said first add/drop path to a drop path; at
least a first optical receiver configured to receive the output
signal from said add/drop device; and, at least a second optical
receiver configured to receive the drop signal from said add/drop
device.
2. The system of claim 1, wherein: said add/drop device includes an
add path configured to provide for add signal wavelengths to pass
to said output path; and, at least a second optical transmitter
configured to transmit information via optical signals including at
least one add signal wavelength to said add path.
3. The device of claim 1, further comprising: a first optical
circulator having at least four ports including an input port
optically connected to a first add/drop port, a second add/drop
port optically connected by a first add/drop path to said first
add/drop port, and to an output port; a second optical circulator
having at least three ports including a first add/drop port
optically connected by a second add/drop path to a second add/drop
port that is optically connected to a drop port; wherein said first
selective element is optically connected between said first
add/drop ports on said first and second circulators; said second
selective element is optically connected between said second
add/drop ports on said first and second circulators; and, said
input path, output path, and drop path are optically connected to
said input port, output port, and drop port, respectively.
4. An optical add/drop device comprising: a first selective element
configurable to pass at least a first signal wavelength from an
input path to a first add/drop path and pass continuing signal
wavelengths differing from the first signal wavelength to a second
add/drop path; and, a second selective element configurable to pass
at least a second signal wavelength from said first add/drop path
and continuing signal wavelengths differing from the second signal
wavelength from said second add/drop path to an output path and
pass the second signal wavelength from said second add/drop path
and signal wavelengths differing from the second signal wavelength
from said first add/drop path to a drop path.
5. The device of claim 4, wherein at least one of the first and
second selective elements is configurable such that the first
signal wavelength becomes equal to the second signal
wavelength.
6. The device of claim 4, further comprising an add path optically
connected to add signal wavelengths to said output path.
7. The device of claim 4, wherein said add path is optically
connected to said first add/drop path via said first selective
element.
8. The device of claim 4, wherein said first selective element
includes a Bragg grating having a central reflective signal
wavelength corresponding to the first signal wavelength.
9. The device of claim 4, wherein said second selective element
includes a Bragg grating having a central reflective signal
wavelength corresponding to the second signal wavelength.
10. The device of claim 4, wherein said first selective element is
one of a plurality of first selective elements configurable to
correspond to different signal wavelengths.
11. The device of claim 4, wherein said second selective element is
one of a plurality of second selective elements configurable to
correspond to different signal wavelengths.
12. The device of claim 4, wherein: said first selective element
includes at least one first multiple signal wavelength selective
element configured to selectively a first set of at least two
consecutive signal wavelengths; and, said second selective element
includes at least one second multiple signal wavelength selective
element configured to selectively pass a second set of at least two
consecutive signal wavelengths, and said first and second selective
elements being configurable to selectively pass the same signal
wavelengths.
13. The device of claim 4, wherein at least one of said first and
second selective elements is configured to selectively pass a
plurality of signal wavelengths.
14. The device of claim 4, wherein said first and second selective
elements include Bragg gratings configured to selectively reflect
at least the first and second signal wavelengths.
15. The device of claim 4, wherein said first and second selective
elements include Fabry-Perot filters configured to selectively
transmit at least the first and second signal wavelengths.
16. The device of claim 4, further comprising an input coupler
having an input port optically connecting said input path and said
first selective element and a first add/drop port optically
connecting said first add/drop path and said first selective
element.
17. The device of claim 4, further comprising an output coupler
having an output port optically connecting said output path and
said second selective element and a first add/drop port optically
connecting said first add/drop path, and said second selective
element.
18. The device of claim 4, further comprising: a first optical
circulator having at least four ports including an input port
optically connected to a first add/drop port, a second add/drop
port optically connected by a first add/drop path to said first
add/drop port, and to an output port; a second optical circulator
having at least three ports including a first add/drop port
optically connected by a second add/drop path to a second add/drop
port that is optically connected to a drop port; wherein said first
selective element is optically connected between said first
add/drop ports on said first and second circulators; said second
selective element is optically connected between said second
add/drop ports on said first and second circulators; and, said
input path, output path, and drop path are optically connected to
said input port, output port, and drop port, respectively.
19. A method of dropping a first signal wavelength to a drop path
comprising: inputting an optical signal including at least one
signal wavelength into an input path; providing a first selective
element configured to pass a first signal wavelength from the input
path to a first add/drop path and pass continuing signal
wavelengths differing from the first signal wavelength to a second
add/drop path; and, providing a second selective element configured
to pass a second signal wavelength from the first add/drop path and
continuing signal wavelengths differing from the second signal
wavelength from said second add/drop path to an output path and
pass the second signal wavelength from the second add/drop path and
signal wavelengths differing from the second signal wavelength
including the first signal wavelength from the first add/drop path
to a drop path.
20. A method of preventing a first signal wavelength from being
dropped to drop path according to the method of claim 19
comprising: adjusting the second selective element to pass the
first signal wavelength from the first add/drop path and continuing
signal wavelengths differing from the first signal wavelength from
said second add/drop path to the output path and pass the first
signal wavelength from the second add/drop path and signal
wavelengths differing from the first signal wavelength from the
first add/drop path to a drop path.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation in part of commonly
assigned U.S. Provisional Patent Application Serial No. 60/101,031
filed Sep. 18, 1998, which is incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED, RESEARCH OR
DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] The present invention is directed generally to optical
transmission systems. More particularly, the invention relates to
adding and/or dropping one or more optical signal wavelengths from
a wavelength division multiplexed (WDM) signal in an optical
communications system.
[0004] The emergence of the Internet as a means for transporting
and accessing data combined with continual growth in traditional
communications has greatly accelerated the need for high capacity
transmission systems. Telecommunications service providers, in
particular, have looked to wavelength division multiplexing (WDM)
to further increase the capacity of their existing systems.
[0005] In optical transmission systems, information is typically
transmitted between central processing centers, or points of
presence, which are used to collect information being
electronically transmitted from a number of smaller distributed
locations. In these systems, it is often desirable and cost
effective to distribute or collect information along the optical
path between the centers without the cost of providing another
central processing center. Optical add/drop ("OAD") devices can be
used at locations in the optical system where the amount of
information being transmitted and received at the location does not
make it economically feasible process all of the information being
transmitted in the system.
[0006] Optical add/drop, or insert/remove, devices are generally
configured to drop/remove one or more predetermined wavelengths
("drop wavelengths") from a WDM signal entering the device and
add/insert the same, or possibly different, wavelengths to the
signal. For example, see U.S. Pat. Nos. 5,283,686, 5,555,118,
5,579,143, 5,600,473, 5,726,785, 5,778,118.
[0007] Many OAD devices include one or more filtering elements,
i.e., Bragg gratings, Fabry-Perot filters, etc., which are used to
either drop signal wavelengths for further processing or merely
filter and remove the signal wavelengths from the transmission
line. OAD devices that include filtering elements, or filters,
allow for the reuse of the filter wavelengths to add wavelengths to
the system.
[0008] In some OAD devices, the filtering elements can be tuned to
vary the wavelength that is being-filtered. For example, the
properties of a Bragg grating element can be varied to change the
central reflective wavelength of the Bragg grating. In lightly
populated WDM systems, sufficient bandwidth exists between signal
wavelengths that the elements can be tuned to a wavelength not
carrying a signal. Thus, a tunable OAD device can be provided in
which the dropped or filtered wavelengths can be variably tuned
according the requirements of the system 10.
[0009] However, in dense wavelength division multiplexing ("DWDM")
systems, there is not sufficient bandwidth between the signal
wavelengths to tune the filtering element in the OAD device to pass
a wavelength without interfering with an adjacent wavelength.
Therefore, many traditional OAD devices can not operated as tunable
devices in DWDM systems.
[0010] This limitation of traditional OAD devices affects the
flexibility of DWDM systems, particularly when planning for system
upgrades or reconfigurations. For example, traditional OAD devices
will have to be replaced when new or different wavelengths are to
be dropped and/or added depending upon the configuration of the
traditional OAD device.
[0011] One proposed solution is to place the filtering elements in
one leg of an optical line switch and provide a bypass line in
another leg of the switch. While bypass line switching provides the
desired functionality, the use of line switches can introduce an
unacceptable amount of loss into the OAD device. As such, the
switched filter OAD devices can not be widely deployed in optical
systems. Thus, there remains a clear need for OAD devices that
provide increased flexibility and inexpensive upgrade paths for
DWDM systems.
BRIEF SUMMARY OF THE INVENTION
[0012] The present invention addresses the need for increasingly
flexible optical transmission systems, apparatuses, and methods
including add/drop and cross-connect devices for WDM systems.
Optical systems of the present invention include an add/drop device
and/or cross-connect device, which can reconfigurably add and drop
signal wavelengths between one or more transmission paths.
[0013] The add/drop device includes a first selective element
configurable to pass a first group of signal wavelengths including
at least a first signal wavelength from an input path to a first
add/drop path and pass continuing signal wavelengths differing from
the first group of signal wavelengths to a second add/drop path. A
second selective element is provided that is configurable to pass a
second group of signal wavelengths including at least a second
signal wavelength from the first add/drop path and continuing
signal wavelengths differing from the second group of wavelengths
from the second add/drop path to an output path. The second
selective element also passes the second group of signal
wavelengths from the second add/drop path and wavelengths differing
from the second group of signal wavelengths from the first add/drop
path to a drop path.
[0014] The add/drop device may also include an add path to
introduce add signal wavelengths directly to the output path or via
at least one of the first and second selective elements. In
addition, a broadcast drop path from the input path can be provided
to access all signal wavelengths entering the OAD device via the
input path.
[0015] In various embodiments, the add/drop device includes first
and second optical circulators. The first optical circulator
includes an input port optically connected to a first add/drop
port, a second add/drop port optically connected by a first
add/drop path to the first add/drop port, and to an output port.
The second optical circulator includes a first add/drop port
optically connected by a second add/drop path to a second add/drop
port, which can be optically connected to a drop port. The first
selective element is optically connected between the first add/drop
ports on the first and second circulators and the second selective
element is optically connected between the second add/drop ports on
the first and second-circulators.
[0016] The reconfigurable add/drop devices of-the present invention
provide flexibility in adding, dropping, and cross-connecting
signal wavelengths in WDM systems. The flexibility is provided
without requiring that new components being installed in the system
or that the system be taken offline to perform the
reconfiguration.
[0017] Accordingly, the present invention addresses the
aforementioned concerns by providing optical systems apparatuses,
and methods having increased flexibility. These advantages and
others will become apparent from the following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Embodiments of the present invention will now be described,
by way of example only, with reference to the accompanying drawings
for the purpose of illustrating embodiments only and not for
purposes of limiting the same; wherein like members bear like
reference numerals and:
[0019] FIGS. 1a&b show optical system embodiments;
[0020] FIGS. 2-9 show various add and/or drop device embodiments;
and,
[0021] FIGS. 10-11 show exemplary pass or stop band configuration
for selective elements.
DESCRIPTION OF THE INVENTION
[0022] Optical systems 10 of the present invention include an
optical add and/or drop ("add/drop") device 12 optically connecting
one or more optical transmitters 14 to one or more optical
receivers 16 via an optical transmission medium 18, which is
usually an optical fiber.
[0023] The optical system 10 can be controlled by a network
management system and configured in multi-dimensional networks
(FIG. 1a) or in one or more interconnected point to point links
(FIG. 1b). The system 10 can be configured to provide
uni-directional or bi-directional transmission in each fiber 18.
Optical amplifiers 19 can also be disposed along the optical fiber
18 to overcome attenuation caused by the fiber 18 or the OAD and
other devices installed along the fiber.
[0024] The optical transmitters 14 are generally configured to
receive information via electrical signals and transmit the
information via optical signals using one or more optical
wavelengths. The optical receivers 16 are generally configured to
receive the optical signals from the transmitter 14 and convert the
information carried by the one or more optical wavelengths into
corresponding electrical signals.
[0025] In FIG. 1(b) embodiments, the system 10 can be configured as
a network in which one or more optical cross-connect switches or
routers 20 can also be used to interconnect diversely located
transmitters 14 or receivers 16. In various embodiments, one or
more of the transmitters 14 and receivers 16 can be wavelength
tunable to provide wavelength allocation flexibility in the optical
system 10.
[0026] One skilled in the art will appreciate that the transmitters
14 and receivers 16 can be externally connected to an electrical
transmission system or be used within a larger optical system for
signal regeneration, such as with back to back terminals that are
either connected directly or via interfacial devices. The
interfacial devices, such as electrical and optical cross-connect
switches, IP routers, etc., provide interface flexibility within,
and at the periphery of, the optical system 10. The interfacial
devices can be configured to receive, convert, and provide
information in one or more various protocols, encoding schemes, and
bit rates to the transmitters 14, and perform the converse function
for the receivers 16. The interfacial devices also can be used to
provide protection switching in various nodes 16 depending upon the
configuration.
[0027] In WDM systems, a plurality of information carrying optical
signal wavelengths .lambda..sub.i provided by one or more
transmitters 14 are combined into a WDM optical signal
.LAMBDA..sub.i using an optical combiner 21 and sent through the
fiber 18. An optical distributor 23 is used to provide one or more
of the signal wavelengths in the WDM signal to the receivers 16.
The optical combiners 21 and distributors 23 can include wavelength
selective and non-selective ("passive") fiber and free space
devices, as well as polarization sensitive devices. Passive or WDM
couplers/splitters, circulators, dichroic devices, prisms,
gratings, etc. can be used alone, or in combination with various
tunable or fixed, high, low, or band pass or stop, transmissive or
reflective filters, such as Bragg gratings, Fabry-Perot devices,
dichroic filters, etc. in various configurations of the optical
combiners 21 and distributors 23. Furthermore, the combiners 21 and
distributors 23 can include one or more serial or parallel stages
incorporating various devices to multiplex, demultiplex, and
broadcast signal wavelengths .lambda..sub.i in the optical systems
10.
[0028] FIGS. 2-9 show exemplary embodiments of the add/drop devices
of the present invention. Generally, an optical signal
.LAMBDA..sub.IN including wavelengths .lambda..sub.1-N from the
transmission fiber 18 enters the OAD device 12 via an input path
22. The optical signal .LAMBDA..sub.IN will generally be a WDM
signal, but it is not necessary in the present invention. The input
path 22 passes the optical signal .LAMBDA..sub.IN to a first set of
selective elements 24.sub.1, which are configurable to pass, or
direct, a first group of signal wavelengths including one or more
first signal wavelengths that are to be dropped, i.e., "1.sup.st
drop wavelengths" .lambda..sub.D1 to a first add/drop path 26.sub.1
and pass any continuing wavelengths differing from the 1.sup.st
drop wavelengths .lambda..sub.C1 to a second add/drop path
26.sub.2.
[0029] A second set of selective elements 24.sub.2 are optically
connected between the first and second add/drop paths, 26.sub.1,
and 26.sub.2. The second selective elements 24.sub.2 are
configurable to pass, or direct, a second group of signal
wavelengths including one or more second signal wavelengths that
are to be dropped, i.e., "2.sup.nd drop wavelengths"
.lambda..sub.D2 from the second add/drop path 26.sub.2 and
wavelengths differing from the second drop wavelengths
.lambda..sub.D2 from the first add/drop path to a drop path 28. In
addition, the second selective elements 24.sub.2 are configurable
to direct the second wavelengths .lambda..sub.D2 from the first
add/drop path and continuing wavelengths differing from the second
wavelength from the second add/drop path to an output path 30. The
drop path 28 and the output path 30 are collectively referred to as
exit paths. In a number of embodiments, the exit paths may be
interchangeable in location depending on whether the selective
elements 26 pass the drop wavelength via reflection or
transmission, as will be further discussed.
[0030] The selective elements 24.sub.i used in the present
invention can include one or more Bragg gratings, each of which can
reflect one or more signal wavelengths. The selective elements
24.sub.i can also be other transmissive/reflective wavelength
selective elements, such as Fabry-Perot and dichroic filters and
other transmissive/reflective filters.
[0031] In configurations with two sets of selective elements,
24.sub.1 and 24.sub.2, the wavelengths that will be dropped from
the device 12 will be those wavelengths that are not common to both
selective elements (.lambda..sub.D1.noteq..lambda..sub.D2).
Likewise, the wavelengths that are common to the two selective
elements, 24.sub.1, and 24.sub.2, will not be dropped, but passed,
or directed, through the device 12 to the output path 30. One
skilled in the art will appreciate that additional add/drop paths
26.sub.i and selective elements 24.sub.i can be cascaded to select
various wavelengths from the input signal .LAMBDA..sub.IN.
[0032] The present invention can be used with selective elements
24.sub.i that are configurable to reflect or transmit two or more
consecutive, i.e., contiguous or side by side, signal wavelengths.
The reflective/transmissive bandwidth of the elements 24.sub.i used
to select two or more wavelengths are larger than the bandwidth
between the signal wavelengths, thus traditional tuning of the
elements can not be performed. However, the present invention
provides for preventing the dropping of multiple signal wavelengths
by tuning the selective elements 24.sub.1, and 24.sub.2, to the
same wavelength.
[0033] FIGS. 2(a&b) show two embodiments in which first and
second optical circulators, 32a and 32b, respectively, are used to
perform the add/drop functions and provide equal loss through all
paths in the device 12. As shown in FIG. 2(a), the optical signal
.LAMBDA..sub.IN enters the OAD device 12 through the input path 22
and into port I of the first circulator 32a. The signal
.LAMBDA..sub.IN exits the first circulator 32a from a first
add/drop port AD1, where it encounters the first selective element
24.sub.1. If the first selective element is a reflective element,
designated 24.sub.R1, the first drop wavelengths .lambda..sub.D1
will be reflected back through the first add/drop port AD1 into
first circulator 32a, travel along the first add/drop path
26.sub.1, exit the first circulator 32a through a second add/drop
port AD2, and encounter the second selective element 24.sub.2.
[0034] Likewise, the continuing wavelengths differing from the
first drop wavelengths .lambda..sub.C1 will pass through the first
selective element 24.sub.R1, enter second circulator 32b through
the first add/drop port AD1 and pass along the second add/drop path
26.sub.2. The continuing wavelengths .lambda..sub.C1 will exit the
second circulator 32b via a second add/port port AD2 and encounter
the second selective element 24.sub.2.
[0035] If the second selective element 24.sub.2 is a reflective
element, designated 24.sub.R2, the second drop wavelengths
.lambda..sub.D2 will be reflected from the continuing wavelengths
.lambda..sub.C1 to the drop path 28 and exit the device 12 through
the second add/drop port AD2 and exit port E of the second
circulator 32b. In addition, any wavelength from the first drop
wavelength .lambda..sub.D1 differing from the second drop
wavelengths .lambda..sub.D2 will pass through the second selective
element 24.sub.R2 and also exit the device 12 through the drop path
28 via the same path.
[0036] Analogously, those continuing wavelengths .lambda..sub.C1
that differ from the second drop wavelengths .lambda..sub.D2 will
pass through the second selective element 24.sub.2 to the output
path 30 exiting the device 12 through the second add/drop port AD2
and exit port E of the first circulator 32a. Also, the second drop
wavelengths .lambda..sub.D2 will be reflected from the first drop
wavelength .lambda..sub.D2 and exit the device 12 through the
output path 30.
[0037] The second selective element 24.sub.2 can be used to drop
additional wavelengths not dropped using the first selective
element 24.sub.1, or prevent the dropping of one or more of the
first drop wavelengths .lambda..sub.D1 Similarly, the selective
elements 26i can be transmissive elements and the paths followed by
the various wavelengths will be reversed, as shown in FIG.
2(b).
[0038] As shown in FIG. 2(a), an add path 31 can also be optically
connected to the second circulator 32b through an input port I to
add an optical signal .LAMBDA..sub.Add. In this configuration, add
wavelengths .lambda..sub.A1 and .lambda..sub.A2 in the optical
signal .LAMBDA..sub.Add corresponding to the first and second drop
wavelengths .lambda..sub.D1 and .lambda..sub.D2 can be inserted
into the optical signal .LAMBDA..sub.out exiting through the output
path 30. In addition, wavelengths in the optical signal
.lambda..sub.Add not corresponding to the first and second drop
wavelengths .lambda..sub.D1 and .lambda..sub.D2 will exit the
device through drop path 28 in optical signal .LAMBDA..sub.Drop.
One skilled in the art will appreciate that the add path 32 can be
included in the device 12 at many different locations, such
proximate the output path 30.
[0039] FIGS. 3(a) & (b) shows an analogous embodiment to FIGS.
2(a) & (b), in which couplers 34 in combination with optical
isolators 36 has been substituted for the optical circulators 32.
FIGS. 4 and 5 show combinations of couplers 34 and isolators 36
with circulators 32. While couplers 34 can be used in the device
12, the couplers can have an excessively high insertion loss. For
example, if 3 dB couplers were employed, half of the signal power
would be lost with each pass through the coupler. In various
embodiments, the optical filter can be included in the coupler, as
described in U.S. Pat. No. 5,457,758, to reduce the loss typically
associated with couplers.
[0040] FIG. 6(a) shows a particular embodiment of the configuration
shown in FIGS. 2(a) in which Bragg gratings are used as the
selective elements 24.sub.1 and 24.sub.2. An additional add path 31
can be coupled to the output path 30 as shown to allow for the
addition of new and/or reused wavelengths to the output path 30 of
the device 12. An additional drop port can also be provided from
the input path 22 to provide broadcast access to all of the
wavelengths before entering the circulator 32a.
[0041] In FIG. 6(b) embodiments, a broadcast and continue drop is
provided before the circulator 32a to provide access to all
wavelengths. Likewise, the add path 31 is coupled to the output
path 30 to allow the addition of new wavelength and the reuse of
wavelengths that were filtered using selective elements 24.sub.1,
and 24.sub.2. In these embodiments, the device 12 serves to filter
noise at various wavelengths before a new signal are added to the
wavelength via the output port 30. The device 12 can also filter
noise or unwanted signal remnants at wavelengths, to which signals
are not being added through the device 12. It will be appreciated
that unwanted signal remnant and noise filters can be included
before, between, and/or after the drop and add paths.
[0042] As shown in FIG. 7, the device 12 can include a 1.times.N
optical splitter or switch 38 in which one of the N output ports is
optically connected to the input path 22 and the remaining port are
optically connected to bypass fibers 40i. The output path 30 is
optically connected along with the bypass fibers to an N.times.1
combiner or switch 42 through which the output signal
.LAMBDA..sub.OUT travels. As previously discussed, add wavelengths
can be inserted into the output path 30 using a coupler 34 or other
appropriate device.
[0043] The switch configuration of FIG. 7 provides for upgrading,
replacing, removing and generally maintaining an in service OAD
device without an extended disruption of communications traffic
passing through the optical system 10. For example, a new OAD
device 12 can be inserted into one of the-bypass fibers 40i, while
traffic is passing through the currently installed OAD device 12.
Following installation of the new OAD device, the switches 38 and
42 can be switched such that traffic passes through the newly
installed OAD device. Alternatively, traffic could be routed
through the bypass fibers 40i to bypass the OAD device until such
time as the device is needed. Also, different OAD devices could be
optically connected to different ports of the switches 38 and 42,
thereby allowing different wavelengths to be dropped depending upon
the OAD device selected.
[0044] The present invention can be extended by cascading
additional selective elements 24.sub.i in the OAD device 12. FIG. 8
shows an embodiment in which a third selective element 24.sub.3 is
included in the device 12 and connected to the circulators 32 via
third add/drop port AD3.
[0045] In addition, other components can be included in the OAD
device 12 to increase the overall functionality. In FIG. 9
embodiments, amplifiers 19, such as erbium doped amplifier (EDFA),
can be added to the device 12 between fourth add/drop ports AD4 of
the circulators 32. In this configuration, the amplifier 19 will
amplify signals passing to both the output path 30 and the drop
path 28 from the input path 22 and the add path 31, thereby
eliminating the need for up to four separate amplifiers.
[0046] Various configurations of the selective elements 24 used in
the OAD device 12 can be provided depending upon the particular
system in which the device 12 is deployed. FIGS. 10 and 11 depicts
two exemplary configurations of the selective elements 24 in the
device 12. In FIG. 10, selective elements 24.sub.1, and 24.sub.2
have pass or stop bands corresponding to .lambda..sub.1 and
.lambda..sub.2, respectively. In the configuration shown in FIG.
10a, the OAD device 12 will drop both .lambda..sub.1 and
.lambda..sub.2. As shown in FIGS. 10b&c, tuning either of the
selective elements 24.sub.1 and 24.sub.2 to the wavelength of the
other selective element can turn off the add/drop functionality of
the device 12.
[0047] The selective elements 24 can also be configured to have
pass or stop bands that include one or more wavelengths and the
same or different bandwidths. In FIG. 11, for example, the
selective elements 24.sub.1 and 24.sub.2 include a plurality of
filters having different bandwidths, which can be tuned to provide
various combinations of add and drop wavelengths. FIGS. 11a&b
show tuning configurations for add/dropping all or none of the four
wavelengths in the example, respectively. FIGS. 11c&d show two
of the other possible combinations.
[0048] While the present invention is particularly useful in DWDM
systems, it can also be deployed in lightly populated WDM systems.
Those of ordinary skill in the art will appreciate that numerous
modifications and variations that can be made to specific aspects
of the present invention without departing from the scope of the
present invention.
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